Vacuum plasma processors include a vacuum chamber containing a workpiece holder, i.e., chuck, for carrying a workpiece having an exposed surface which is plasma processed, i.e., a surface which a plasma etches and/or on which a plasma deposits materials. The etching and depositing are achieved inter alia by ions in a low impedance plasma in the chamber resulting from introducing one or more suitable gases into the chamber and the application of an r.f. field to the gas.
The workpiece temperature is controlled by applying an inert heat transfer gas, such as helium, to the back face of the workpiece. The heat transfer gas improves thermal contact between the workpiece and the chuck which is cooled by water. Typically, the workpieces are relatively thin substrate plates made of electrical conducting materials (i.e., metals), semiconductors or dielectric glass sheets. The workpiece must be clamped to the chuck to hold the workpiece in place against the pressure of the heat transfer gas pushing on the workpiece back face.
Vacuum plasma processing of dielectric glass sheet workpieces has assumed great importance with the introduction of flat panel displays having panels with relatively large dimensions, measured in feet. The co-pending, commonly assigned application of Shufflebotham et al., Ser. No. 08/542,959, now U.S. Pat. No. 5,847,918, filed Oct. 13, 1995, entitled "Electrostatic Clamping Method and Apparatus for Dielectric Workpieces in Vacuum Processors," discloses clamping a dielectric workpiece to an electrostatic chuck in a vacuum plasma processor chamber having a wall at a reference potential. To clamp a dielectric glass workpiece, the low impedance plasma must be incident on a surface of the workpiece exposed to the plasma while a relatively high voltage is applied to a metal electrode plate of the chuck. The electrode is at a voltage substantially different from the plasma while electrostatic charge at approximately the reference potential is applied to the workpiece exposed surface by the low impedance plasma. An electrostatic force is thereby developed between the workpiece and the chuck to clamp the workpiece to the holder.
In a monopolar electrode arrangement disclosed in the aforementioned application, a DC voltage (preferably, but not necessarily, negative) is applied to a metal electrode plate, thence to the workpiece, preferably through a thin protective insulator, preferably formed as an anodized layer on the metal electrode plate. The negative voltage attracts from the plasma net positive charge which settles on the top face of the glass. The attractive force between negative charges on the top of the metal electrode and positive charges on the top of the glass substrate (i.e., workpiece) clamps the glass. Hence, sufficient electrostatic clamping force to hold the substrate on the chuck is applied through the thickness of the workpiece by a voltage difference between the DC voltage applied to the electrode and the approximately DC reference voltage on the workpiece exposed face.
The method and apparatus disclosed in the aforementioned application have been successfully used in many commercial situations for glass dielectric workpieces, such as panels used in flat panel displays. However, when the method and apparatus of the aforementioned application were used in processing glass dielectric workpieces of certain flat panel display manufacturers, the workpieces stuck to the electrostatic chucks and could not be released, i.e. dechucked. Dechucking is performed by lifting the workpiece from the chuck with a mechanical mechanism, usually including pins that are lifted through the chuck into engagement with the bottom face of the workpiece. The mechanism must exert a greater force on the workpiece than the force the chuck applies to the workpiece to release the workpiece from the chuck. However, the force exerted by the workpiece must be low enough to prevent workpiece damage, e.g., breaking, cracking or permanent bending.
Originally the failure to dechuck was thought to be due to a malfunction in the electrostatic chuck. However, when the same glass workpiece was put into a processor having a chuck known to function satisfactorily, the workpiece could still not be dechucked. Our investigations led to the realization that the glass workpiece that could not be dechucked had different passive electric characteristics from the glass workpieces which were successfully dechucked from the prior art electrostatic chucks. The present invention is based, inter alia, on the realization that different glass workpieces used as flat panel displays have different passive electric characteristics, particularly resistivity, which affect dechucking from electrostatic chucks.
Problems have been encountered in the past in dechucking semiconductor wafers from electrostatic chucks. The wafer chucking structures for glass and semiconductor workpieces are similar in that both include a metal electrode covered by an insulator which carries the workpiece. Space in the vacuum between non-contacting portions of the workpiece bottom face and the insulator top face, referred to as a Johnsen-Rahbek gap, enters into an analysis of mechanisms for chucking the different types of workpieces. While there are several ways a silicon wafer can become stuck to an electrostatic chuck, only one (wherein a residual charge left on a capacitor formed between the wafer bottom face and the top face of the insulating layer) is believed relevant to the discussion of the background of the present invention.
The mechanisms for chucking a semiconductor workpiece are substantially different from those of a dielectric workpiece. The mechanisms are such that a substantially higher voltage must be applied to an electrostatic chuck for a dielectric workpiece than is applied to an electrostatic chuck for a semiconductor wafer. Electrostatic chucks for high resistivity glass substrates require higher voltage than electrostatic chucks for silicon wafers because a silicon wafer clamps due to a mutual attraction of opposite polarity charges on the top of the electrode and the bottom of the wafer. These charges are separated by a relatively thin dielectric layer on the chuck electrode. A glass panel, on the other hand, is held in place by a mutual attraction of charges on the top of the chuck electrode and the top of the glass panel. These charges are separated by the thickness of the glass panel, which is much greater than the thickness of the dielectric coating on the electrode. Because of the greater thickness of the dielectric forming the capacitor between the electrode and the top face of the dielectric sheet being chucked than the thickness of the capacitor between the electrode and the bottom face of the relatively low resistivity semiconductor wafer, greater chucking voltage is required for the glass panel than for the wafer.
A negative voltage applied to the metal electrode of a chuck for a semiconductor wafer attracts positive charge from the plasma. The positive charge settles on the exposed top wafer face. While semiconductor materials have a resistivity that is high compared to the resistivity of a metal, the resistivity of a semiconductor material is much lower than that of a dielectric, such as the glass of a flat panel display. The maximum resistivity of a silicon wafer is about one ohm meter so the time constant for positive charge to move between top and bottom faces of the wafer is less than about one microsecond. Since one microsecond is a much shorter time than the typical time required for wafer processing, this charge movement through the wafer is considered to occur instantaneously.
Similarly, there is a tendency for charge on the exposed top surface of a high resistivity glass dielectric sheet to migrate through the glass from the top face of the glass to the glass bottom face. Since the resistivity of the high resistivity glass is much higher than the resistivity of silicon wafers (greater than about 10.sup.15 ohm meters, compared to about one ohm meter) the time scale for the charge migration through the high resistivity glass is about a week. Because the charge migration time through the high resistivity dielectric is much longer than a typical processing time for a glass workpiece, the charge migration through the high resistivity glass can be ignored.
In both of these situations (semiconductor wafers and high resistivity dielectric workpieces) chucking and dechucking of the workpiece occurs for time scales (typically less than one or two seconds) determined by the ability of a source of DC voltage applied to the chuck electrode to provide the necessary charge. Chucking and dechucking of the semiconductor and high resistivity glass workpieces thus does not depend on the very fast or very slow time scales of charges moving between top and bottom faces of the workpieces. In both cases, the charge distribution between the top and bottom faces of the workpiece can be considered as fixed during workpiece processing time. As discussed infra, the charge distribution between the top and bottom faces of dielectric workpieces having low and intermediate resistivities (i.e., between about 10.sup.8 and 10.sup.14 ohm meters) cannot be considered as fixed during workpiece processing.
Birang et al., U.S. Pat. No. 5,459,632, which appears to have the same disclosure as Birang et al., U.S. Pat. No. 5,612,850, discloses a semiconductor wafer dechucking method wherein a dechucking voltage applied to a monopolar chuck electrode has the same polarity as the polarity of the voltage used to maintain the workpiece in a chucked position. The dechucking voltage has a magnitude different from the chucking voltage to minimize the electrostatic attractive force between the chuck and workpiece. An "optimum" value for the dechucking voltage is determined empirically or by monitoring the amplitude of a current pulse produced as the workpiece is initially mounted on the chuck.
Monitoring the amplitude of the current pulse which flows through the workpiece and the electrostatic chuck when the workpiece is first applied to the chuck is not applicable to processing of glass, dielectric workpieces. This is because no current flows between the electrostatic chuck and the dielectric workpiece when the workpiece is initially placed on the chuck. For glass panels, there is no current pulse when a new panel is first lowered onto the electrostatic chuck. This is because any residual sticking charge on the previously processed dielectric workpiece left with that dielectric workpiece when it was removed from the electrostatic chuck.
As pointed out in Birang et al., U.S. Pat. No. 5,491,603, the method disclosed in the other two Birang et al. patents requires sophisticated measurements of a very short duration electrical pulse. To avoid such sophisticated measuring procedures, the '603 patent discloses a somewhat complex method of calculating the "optimum" voltage by applying an electrostatic potential to the chuck, then introducing a gas between the wafer and chuck and then reducing the electrostatic potential of the chuck while observing a rate of gas leakage from between the wafer and chuck. The optimum dechucking voltage is recorded in a memory as the value of electrostatic potential that occurs when the leakage rate exceeds a predetermined threshold. The calculated optimum voltage is apparently applied to the chuck as or after the plasma is turned off; after the plasma is turned off the wafer is lifted from the chuck. There is no disclosure in the '603 patent of controlling the chucking voltage applied to the chuck during wafer processing in response to the flow rate of gas applied to the workpiece via the chuck during processing. The '603 patent also has no disclosure of maintaining the force applied by the chuck to the wafer substantially constant during wafer processing by a plasma.
In all three Birang et al. patents, which are directed to monopolar chucks for semiconductor wafers, the semiconductor wafer conducts current and trapped charge responsible for a residual clamping force between the workpiece and the electrostatic chuck is conducted through a vacuum gap between the workpiece and top surface of a chuck dielectric layer. Because of the semiconductor wafer conducting properties, the trapped charge which holds the workpiece to the chuck subsists between the workpiece bottom face and the chuck electrode. In contrast, in a glass dielectric workpiece, most of the charge is trapped in the workpiece itself, between the top and bottom faces of the glass dielectric workpiece.
Watanabe et al., U.S. Pat. No. 5,117,121 discloses a method of releasing a semiconductor wafer from a bipolar electrostatic chuck. To clamp the semiconductor wafer workpiece to the bipolar chuck, a DC voltage having a predetermined amplitude and polarity is applied between two chuck electrodes. Because the Watanabe et al. workpiece is a semiconductor wafer, the vast majority of the trapped charge is between the wafer bottom face and the chuck electrode; virtually no charge is trapped between the opposite faces of the workpiece.
After clamping by the DC voltage and before the workpiece is removed from the chuck, a second voltage having a polarity opposite to the polarity of the first voltage is applied to the chuck electrodes to eliminate a residual attractive force which the chuck is applying to the semiconductor workpiece. The second voltage has an amplitude which is one-and-a-half to two times higher than the amplitude of the voltage of the first polarity. The second voltage is continuously applied to the bipolar electrodes for a time period inversely proportional to the amplitude of the second voltage. Apparently, the amplitudes of the first and second voltages are empirically determined. In any event, the amplitudes of the first and second voltages are not determined by an analysis of the passive electric impedance properties of a dielectric, glass workpiece.
It is, accordingly, an object of the present invention to provide a new and improved method of and apparatus for electrostatically chucking and dechucking workpieces in a vacuum plasma processor.
Another object of the invention is to provide a new and improved apparatus for and method of electrostatically chucking and dechucking dielectric glass workpieces, particularly adapted to be used as flat panel displays, in a vacuum plasma processor, wherein during processing, the clamping force exerted by the chuck on the workpiece is sufficient to overcome the tendency of the workpiece to be moved relative to the chuck by cooling gas flowing through the chuck to the workpiece.
A further object of the invention is to provide a new and improved electrostatic chuck apparatus and method used in a vacuum plasma processor for dielectric, glass workpieces of all types, wherein throughput of the workpiece is increased because the workpieces are quickly dechucked.
Still another object of the invention is to provide new and improved methods of dechucking glass, dielectric workpieces from an electrostatic chuck based on an analytic analysis of the workpiece passive electric parameters.
Yet an additional object of the invention is to provide a new and improved method of and apparatus for determining the amount of charge to be removed from a workpiece during dechucking from an electrostatic chuck.